1. Field of the Invention
The present invention relates generally to the design, construction and operation of combustion air preheating systems with gas-to-gas heat exchangers. More particularly, the present invention is a heat exchange system which is suitable for preheating combustion air prior to introduction into conventional air heaters which utilize heat from hot flue gases to heat combustion air.
The combustion efficiency of fossil fuel burning equipment (e.g., a burner) can be increased by heating combustion air prior to introduction to the burner. This is typically achieved by heat exchange between the hot flue gases discharged from fossil fuel burning equipment and the cold inlet combustion air. For example, rotary air heaters (regenerative) and tubular air heaters (recuperative) may be used in this application.
Flue gases resulting from the combustion of many fossil fuel sources, such as high sulfur coals and fuel oils, can possess relatively high levels of acid gases, such as sulfur trioxide. The acid gases remain in the vapor phase so long as the temperature of the flue gas remains above the acid dew point. When the temperature falls below the dew point, however, the acid gases condense, typically forming sulfuric acid.
Condensation of acid gases within combustion air heaters has been problematic. Particularly during cold weather, the ambient air can extract sufficient heat from the flue gases so that the flue gas temperature falls below the dew point in the air heater. As most heat exchangers utilized in this application are constructed from carbon steel, the condensation of sulfuric acid can cause severe corrosion problems. Moreover, fly ash will adhere to the heat exchange surfaces which are wetted with the condensed acid, eventually fouling the passages in the heat exchanger.
To overcome this problem, combustion air preheaters using low pressure steam are usually provided to raise the temperature of the incoming ambient air sufficiently so that the temperature of the flue gas will not fall below the acid dew point. In particular, the temperature is raised sufficiently so that the "cold-end average temperature" is raised to within the range from about 160.degree. F. to 185.degree. F. The cold-end average temperature is defined as the average between the air inlet temperature to the air heater (i.e., the temperature from the air preheater) and the temperature of the cooled flue gases as they pass out of the air heater.
While the use of air preheaters to raise the cold-end average temperature is generally workable, the use of steam heat exchangers is often unreliable. Such steam heat exchangers are subject to freezing, leaking, and condensate contamination. To overcome these problems, many steam preheaters require circulation of hot water or a glycol through the tubes, which substantially complicates their design and operation. The use of glycol eliminates the freezing problem, but can result in corrosive oxidation products which require that the system be blanketed with nitrogen.
For these reasons, it would be desirable to provide improved methods and systems for preheating combustion air in order to raise the cold-end average temperature of the air entering the combustion air heater. Such systems should be of simple design, be inexpensive to build and to operate, and should be of compact size so that they may be retrofitted onto existing combustion systems in place of steam preheaters of the type just described.
2. Description of the Background Art
U.S. Pat. No. 4,483,391, describes a two stage preheater for combustion air where heated combustion air is recirculated to preheat the incoming combustion air which is in thermal contact with the lowest temperature flue gas. U.S. Pat. Nos. 4,416,325; 4,213,404; 4,037,543; 3,678,869; 3,509,834; 2,544,369; 2,128,431; and 2,088,225; all relate to the construction of gas-to-gas heat exchangers. An article entitled "Air Heaters and Economizers," which appeared in Power Magazine, June 1988, pages B-68 to B-70, describes the construction of convention heat exchangers which are used for preheating combustion air in fossil fuel boilers.